1. Field of the Invention
The subject matter of the invention is an arrangement for aftertreatment of exhaust gas in lean-burn internal combustion engines such as diesel engines and Otto engines with direct injection.
2. Description of the Related Art
In order to adhere to the legally prescribed limits on exhaust gas, virtually all lean-burn internal combustion engines have been outfitted in the meantime with catalytic aftertreatment systems such as:
NOX storage catalyzers
SCR catalyzers, or
particulate filters.
When a NOx storage catalyzer is used, the combustion changes constantly between overstoichiometric combustion and substoichiometric combustion. In the lean operating phases, the nitrogen oxides are stored in the form of nitrates which are reduced to nitrogen in the rich operating phases by means of carbon monoxide and hydrocarbons. The storage in the form of nitrate proceeds by way of NO2 which accumulates in the form of nitrate on the barium or calcium storage components.BaCO3+2NO2+0.5O2Ba(NO3)2+CO2  (1)
The conversion of the stored nitrate takes place subsequently in the rich operating phases by means of carbon monoxide, hydrocarbons (HC) and hydrogen on platinum and/or ruthenium as active component(s).Ba(NO3)2+COBaCO3+2NO+O2  (2)2CO+2NO2CO2+N2  (3)HC+2NO+O2+0.5H2+COH2O+2CO2+N2  (4)
The NO2 is needed for this process is formed at NO oxidation catalyzers, usually containing platinum, and/or at the platinum-containing NOx storage catalyzers by means of oxygen which is contained in the exhaust gas and formed from the nitrogen monoxide emitted by the engine.2NO+O22NO2  (5)
The problem with this NO oxidation is that the maximum NO2 proportions that can be achieved are limited thermodynamically at high temperatures. As a result, in contrast to other exhaust gas catalyzers, the desired conversions will decrease again at high temperatures after an increase at low temperatures and there will not be a pronounced plateau-like conversion maximum.
Particle separators, as they are called, or particulate filters are used in power plants and in vehicles to minimize fine particles. A typical arrangement with particle separators for use in vehicles is described, for example, in EP 1 072 765 A1. Arrangements of this kind differ from those using particulate filters in that the diameter of the channels in the particle separator is substantially greater than the diameter of the largest occurring particle, while the diameter of the filter channels in particulate filters is in the range of the diameter of the particles. Due to this difference, particulate filters are prone to clogging, which increases the exhaust gas back pressure and reduces engine performance. An arrangement and a method with particulate filters are shown in U.S. Pat. No. 4,902,487. A distinguishing feature of the two above-mentioned arrangements and methods consists in that the oxidation catalyzer—usually a catalyzer with platinum as active material—arranged upstream of the particle separator or particulate filter oxidizes the nitrogen monoxide in the exhaust gas by means of the residual oxygen that is also contained to form nitrogen dioxide which is converted in turn in the particle separator or particulate filter with the carbon particles to form CO, CO2, N2 and NO. In this way, a continuous removal of the deposited solids particles is carried out. Accordingly, regeneration cycles that must be carried out uneconomically in other arrangements are dispensed with.2NO2+C2NO+CO2  (6)NO2+CNO+CO  (7)2C+2NO2N2+2CO2  (8)
In order to meet future exhaust gas regulations, it will be necessary to use arrangements for reducing nitrogen oxide emissions and arrangements for reducing fine particles emissions at the same time.
As was already mentioned, the NO2 needed for the storage of the nitrogen oxides in NOx storage catalyzers is formed at NO oxidation catalyzers usually containing platinum and/or at the platinum-containing NOx storage catalyzers themselves. In actual engine operation, however, sulfurization of the NOx storage catalyzers due to sulfur contained in the fuel and/or in the engine oil poses a problem. Owing to the combustion, SO2 is formed from this sulfur and is oxidized at the NO oxidation catalyzers downstream to form SO3.S+O2SO2  (9)2SO2+O2SO3  (10)
In this connection, it has been shown that the amount of SO3 which is formed and the amount of NO2 which is formed are directly related; this means that a catalyzer forming large amounts of NO2 generates large amounts of SO3 at the same time. This SO3 forms very stable sulfates with the storage components of the NOx storage catalyzer, which leads to a considerable decrease in the NOx storage capacity and, therefore, to reduced catalyzer performance. A regeneration of the catalyzers can be carried out by actively increasing the exhaust gas temperatures to greater than 900° C., but this can result in thermal damage to the NOx storage catalyzer. Further, the temperature increase is usually connected to an increase in fuel consumption.